R. Ravi

Self-diffusion in liquid metals

Abstract Self-diffusion in liquid metals is investigated using the hydrodynamic model based Stokes–Einstein equation, and the hard sphere model. Based on a study of sixteen liquid metals over a range of temperatures, it is found that the Stokes–Einstein predictions are satisfactory provided a temperature dependent atomic diameter is used. Attempts to fit the self-diffusivity data to Dμ p / T =a constant using the temperature independent Goldschmidt diameter yielded values of p ranging from 0.62 to 1.56 for the sixteen metals studied. Extensive calculations were performed in the case of liquid sodium based on the hard sphere model using three different expressions, based on the Enskog theory and corrections to it, for the diffusion coefficient. The effective hard sphere diameter appearing in these expressions was calculated as per the three well-known prescriptions based on perturbation theory and using two different inter-particle potentials. The hard sphere model, apart from not providing satisfactory numerical accuracy, predicts a much weaker temperature dependence of the self-diffusion coefficient than is observed experimentally.

Characterization of solvent clusters in a supercritical Lennard‐Jones fluid

We have developed a new methodology for characterizing the solvent cluster structures which occur in a pure supercritical fluid in its compressible regime. This methodology takes advantage of the time scale separation which exists between collective‐cluster and individual‐solvent–atom motions in order to classify atoms according to their ‘‘instantaneous’’ local environments. The resultant picture is of a fluid having density inhomogeneities on a mesoscopic length scale—i.e., clusters and cavities. Calculation of partial radial distribution functions shows that atoms residing in different density domains have very different equilibrium structural properties, information which is not available from the usual total radial distribution function. For example, for a 2‐dimensional Lennard‐Jones fluid at a reduced temperature Tr=1.06 the nearest neighbor coordination number in a high density domain is 4.2, whereas in a low density domain it is only 1.0. We have also found that, for such clustering fluids that in ...

Translational and rotational dynamics in liquids. comparison of experiment, kinetic theory and hydrodynamics

Abstract Experimental translational and rotational diffusion data for about 30 molecules dissolved in n -hexane are compared with the predictions of slip hydrodynamics as well as the hard body kinetic theories of Enskog and Evans. The results indicate that slip hydrodynamics and Enskog theory are generally successful in quantitatively predicting both translational and rotational diffusion coefficients while Evans generalization of Enskog theory appears to overestimate the effects of correlated collisons.

Calculation of self-diffusion coefficients in liquid metals based on hard sphere diameters estimated from viscosity data

Abstract Temperature-dependent hard sphere diameters are determined from viscosity data for sixteen liquid metals. These diameters are then used to calculate diffusion coefficients using the hard sphere model as well as the Stokes–Einstein model. Both models predict a temperature dependence of the diffusivity that is weaker than that observed experimentally. However, the predictions are comparable to those obtained using the Protopapas–Parlee hard sphere diameters which were determined from the diffusivity data.

A RIGOROUS ANALYSIS OF McCABE-THIELE ASSUMPTIONS AND THEIR CONSEQUENCES: CRITICAL ROLE OF PARALLEL ENTHALPY LINES

ABSTRACT The assumptions and results associated with the McCabe-Thiele method of distillation column analysis are brought together within a rigorous analytical framework. Within this framework, the assumption that the saturated liquid and vapor enthalpy curves are parallel lines, apart from adiabatic and isobaric operation of the distillation column, is shown to be sufficient to derive all the major results of the McCabe-Thiele method including the constant molar overflow condition. Then, using a rigorous thermodynamic analysis, a comprehensive set of conditions sufficient for the enthalpy curves to be parallel lines are obtained. Some common systems for which these assumptions hold are discussed. Finally, the conditions under which McCabe-Thiele-like methods occur in other separation processes are outlined, with pointers to a unified picture of separation processes.

THE DEFINITION OF REACTION RATE: A CLOSURE

An unambiguous treatment of the rate of chemical transformation for homogeneous reactions is sought by pointing out the key elements of any mathematical theory of physicochemical phenomena and relating them to some of the concepts of reaction engineering. Within this framework, the rate of generation per unit volume of a species through a reaction step emerges as a primitive. The status of the species balance equation as a law(rather than a definition) and that of commonly used rate expressions as constitutive relations are brought forth. These concepts are illuminated using analogies from classical mechanics and thermodynamics.

Binding Energies of H and Cl on Si(111)

Chemisorption or energetic absorption is the first in the sequence of steps that take place at a substrate surface. The change in energy that accompanies chemisorption, called the chemisorption energy, of a species characterizes the binding strength of that species to the substrate. A knowledge of this quantity is essential for predicting the course of surface reactions. Yet very few estimates of this quantity are available for silicon surfaces. In this work, we propose a surface molecule model for chemisorption on silicon

MATHEMATICAL TREATMENT OF THE LOSCHMIDT TUBE EXPERIMENT: SOME CLARIFICATIONS

The mathematical problem associated with the Loschmidt tube experiment is considered with the specific purpose of resolving the discrepancies that exist in the literature regarding the solution to the problem. An outline of one method of obtaining the correct solution is presented using standard techniques.

The optimal use of entropy and enthalpy

Chemical engineers juggle entropy and enthalpy changes to produce chemicals with the minimal expenditure of work. In a free market, the prices of common chemicals correlate very well with the thermodynamic work required to produce them!

Unimolecular Decomposition of Phosphine in Chemical Vapor Deposition Reactors

Unimolecular decomposition of a source species is often the first step in the sequence of reactions in the gas phase and has been found to be of primary kinetic significance in CVD systems. Here, we compute the rate constants for phosphine decomposition under both polysilicon and epitaxial silicon deposition conditions; rate constant values are provided for decomposition at 900 and 1300 K in a pressure range of 0.2-760 Torr